Capacitive sensing is a technology based on capacitive coupling that is used in many different types of sensors, including those to detect and measure proximity, position or displacement, humidity, fluid level, and acceleration. Capacitive sensing may be used as a human interface device (HID) technology, for example to replace the computer mouse. Capacitive touch sensors are used in many devices such as laptop trackpads, digital audio players, computer displays, mobile phones, mobile devices, tablets and others. Capacitive sensors are advantageous for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches. Technologies such as multi-touch and gesture-based touch screens (i.e., touch pads) are also premised on capacitive sensing.
Mutual capacitive sensors have a capacitor at each intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus near the surface of the sensor changes the local electric field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.
Self-capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load on each column or row is measured using current sensing. This produces a stronger signal than mutual capacitance sensing, but it is unable to resolve accurately more than one finger, which results in “ghosting”, or misplaced location sensing
The performance of devices employing mutual-capacitance scanning systems may be impacted by the phenomenon of negative touch (also known as anti-touch). Negative touch is a device state that occurs when s sensor scan is “initialized” (meaning that baseline measurements are made) while touches are active on the surface. For the surface portions where the touches are present, a ‘negative’ effect is created whereby those portions are initialized in a ‘no-touch’ state with a distorted baseline reading. When the actual touch is removed, a ‘touch’ state is indicated on that portion of the surface because a large deviation from baseline is measured. This leads to ‘stuck touches’ and ‘dead zones’ on the surface.